1. Field of the Invention
This invention relates to a wrapping system for stretch-wrap plastic film and in particular linear low density polyethylene films.
2. Discussion of the Background
Stretch-wrap plastic film has been widely used for some time for wrapping and securing a load on a pallet for example. More recently the provision of a pre-stretched wrapping film has been proposed. The use of a pre-stretched wrapping film advantageously uses dramatically less material. The processes and apparatus for producing such a pre-stretched plastic film for wrapping have not however been entirely successful and such pre-stretched films have not gained widespread acceptance in use.
The present invention is directed towards providing an improved wrapping system for stretch-wrap plastic film which is simple and efficient to use.
According to the invention there is provided a wrapping system for stretch-wrap plastic film, comprising:
partially pre-stretching a plastics film by a desired amount,
relaxing the film, and
further stretching the partially pre-stretched film immediately prior to using the pre-stretched film for wrapping.
Advantageously the second stretching of the film immediately prior to application reactivates the memory of the plastic material causing shrinkage after application to firmly wrap and secure a load on a pallet for example.
In a preferred embodiment after relaxing the film, the film is wound on a core, the further stretching being carried out as the film is drawn from the core for use.
The ratio of the elongations of the film during the pre-stretching step and during the further stretching step is about 3:1.
In one embodiment of the invention the total stretching of the film is between 150% and 250%. Most preferably the total stretching of the film is about 200%.
Preferably the pre-stretching and relaxing elongates the plastics film by about 140-160% and the final stretching of the film immediately prior to application gives about a 15-20% further elongation of the pre-stretched film. It has been found that this controlled double stretching of the plastics film gives better results than the prior art in which the plastics film is fully stretched in one step.
In a preferred embodiment the pre-stretching of the plastics film is carried out in a number of steps. Ideally the pre-stretching is carried out in three steps giving in step 1 about 100% elongation, in step 2 about 30% elongation and in step 3 about 20% elongation.
In a further embodiment the film is stretched by feeding the film from a supply roll around two or more spaced-apart film stretching rollers, each roller rotating at a greater speed than the roll or roller immediately upstream to stretch the film between the rollers.
In a particularly preferred embodiment the pre-stretched film is loosely wound onto the core. This advantageously prevents collapse of the core due to the shrinkage in the film after winding. Preferably the film is loosely wound onto the core by feeding film off the final stretching roller at a speed greater than the speed at which the film is wound up on the core.
A further disadvantage with existing pre-stretched wrapping film systems is that when the film is pulled from the core for wrapping edge hang-ups tend to cut the film if it is drawn off the core at an angle which is often the case when wrapping a pallet or the like. Edge hang-ups are generated because the pre-stretched film is extremely thin and is not a problem generally with conventional stretch film. This problem can be overcome according to the invention by leading the film off the core whilst maintaining the line of take-off where the film parts from the core substantially parallel to the central rotational axis of the core. Thus the film is evenly drawn oft the core initially prior to application about the goods to be wrapped.
Preferably the film is drawn from the core around a take-off roller which rotatably engages against an outer surface of the core. This ensures that the film is evenly drawn from the core and it can be then applied at any desired angle onto the materials to be packaged downstream of the take-off roller.
In a preferred embodiment the final stretching of the film is carried out by leading the film from the core around a final stretch roller, the speed of the final stretch roller being greater than the speed of discharge of film from the core.
Conveniently the final stretch roller is drivably connected to the take-off roller through a gear drive which ensures rotation of the final roller at a desired speed greater than the take-off roller to achieve the desired final stretch of the film. Alternatively the final stretch roller may be geared to a rotational support for the core to achieve the same result.
In another aspect the invention provides a method for producing a partially pre-stretched plastics film for subsequent stretching and wrapping about a load.